1. Field of the Invention
The present invention relates to a method for processing a sidewall oxide layer of a shallow trench isolation and densifying the oxide in a shallow trench isolation, and more particularly to a method for reducing a stress and encroachments of a sidewall oxide layer of a shallow trench isolation.
2. Description of the Related Art
As the density of integrated circuits increases, the dimension of an isolation region between active regions in semiconductor devices decreases. With this trend, the conventional local oxidation of silicon (LOCOS) method for isolating active regions, which forms a field oxide layer by using a thermal oxidation technique, confronts the limit in the effective isolation length, thereby degrading characteristics of the isolation region. Furthermore, the conventional LOCOS method possesses some inherent drawbacks resulting from the processes, i.e., lateral oxidation of the silicon underneath the silicon nitride mask, making the edge of the field oxide resemble the shape of a bird""s beak.
According to the disadvantages for LOCOS isolation structures mentioned above, an isolation technique using trenches has been developed. Generally, the trench isolation includes the steps of etching a silicon substrate to form a trench, depositing a oxide layer by using a chemical vapor deposition (CVD) process to fill up the trench, providing the oxide layer a planarized surface by using a chemical mechanical polish (CMP) process, and removing the oxide layer upon the active regions.
According to the technique, the semiconductor substrate is etched at a predetermined depth, thereby providing excellent characteristics of the device isolation. Furthermore, the field oxide layer is formed by using a CVD technique, so that the device isolation region that is defined by a photolithography process can be maintained throughout. The device isolation technique set forth is also known as shallow trench isolation (STI) processes. However, conventional shallow trench isolation processes still have several drawbacks. FIG. 1 shows a cross-sectional diagram of a shallow trench isolation amid a STI conventional process. A silicon substrate 100, a silicon dioxide layers 102, a silicon nitride layer 104 and a silicon dioxide layer 108 are shown in FIG. 1. A sidewall oxide layer 106 is formed over the trench by conventional oxidation processes such as dry or wet thermal oxidation. The sidewall oxide layer 106 is used to eliminate etching induced damage and reduce stress resulting from the following filling of the silicon dioxide 108 layer by conventional chemical vapor deposition. To further reduce the stress, a sidewall reoxidation is usually performed. The conventional sidewall reoxidation is usually a wet oxidation which would result in additional stress and encroachment problems. An encroachment 107 or a bird""s beak is shown in FIG. 1. The encroachment 107 is formed because H2O molecules and O2 always diffuse into the interface of the silicon substrate 100 and the silicon dioxide layer 102. The large stress and the encroachment 107 usually presents defects in neighboring active regions. The defects will result leakage current and degrade the reliability of neighboring devices.
In view of the drawbacks mentioned with the prior art process, there is a continued need to develop new and improved processes that overcome the disadvantages associated with prior art processes. The requirements of this invention are that it solves the problems mentioned above.
It is therefore an object of the invention to provide a processing method for reducing the stress and preventing the encroachment of a sidewall oxide layer of a STI.
It is another object of this invention to provide a STI process which can assure the electrical property of the active regions.
It is a further object of this invention to provide a reliable STI process which can assure the isolation quality between the active regions.
It is another object of this invention to densify the oxide in a shallow trench isolation to prevent the oxide from being lost in the following clean process.
To achieve these objects, and in accordance with the purpose of the invention, the invention uses a method comprising: providing a substrate having a first dielectric layer thereon and a second dielectric layer over said first dielectric layer; forming a trench into said substrate; forming a sidewall oxide layer on the sidewall and bottom of said trench; filling said trench with a dielectric material; and performing an in situ steam generated process comprising introducing hydroxyl to reoxidize said sidewall oxide layer and densify the oxide in a shallow trench isolation.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.